1. Field of the Invention
The invention relates to a crystal oscillator, which performs temperature compensation, or a crystal oscillator, which does not perform temperature compensation, and particularly relates to a crystal oscillator capable of compensating a frequency drift characteristic that varies due to heat.
2. Description of the Related Art
[Conventional Technology]
The conventional temperature compensated crystal oscillator has a temperature compensation circuit in a voltage-controlled oscillator thereof to reduce frequency variation caused by the change of the surrounding temperature. Moreover, crystal oscillators without temperature compensation circuits have also been provided. The frequency of the conventional crystal oscillator may drift (vary or stray). In particular, the frequency drift characteristic thereof may change due to the heat caused by power source variation.
[Frequency Drift Characteristic: FIG. 10]
A frequency drift characteristic is shown in FIG. 6 and FIG. 10. FIG. 6 is a diagram illustrating the variation of a frequency relative to time, and FIG. 10 is a diagram illustrating frequency/power relative to time, which both show the frequency drift characteristic (power coefficient). As shown in FIG. 6, it is known that, as the power source voltage becomes larger, power consumption increases, heat generation increases, and downward drifting is also large (i.e. deterioration of the frequency drift characteristic). In addition, it should be noted that, under various power conditions (that is, under various heat generations), as shown in FIG. 10, the same structures have the same frequency drift characteristic electric coefficient, and the frequency drift characteristic has a proportional relationship with power consumption.
[Related Art]
JP 10-224148 “Piezoelectric Oscillator” (TOYO Communication Equipment Co., Ltd.; referred to as “Patent Reference 1”); JP 02-100502 “Voltage Controlled Oscillator” (Murata Manufacturing Co., Ltd.; referred to as “Patent Reference 2”); and JP 11-186843 “Stabilized Oscillation Circuit” (Toshiba Microelectronics Corp.; referred to as “Patent Reference 3”) are provided as conventional arts related to this field.
Patent Reference 1 discloses a voltage-controlled piezoelectric oscillator, including an amplifier formed by transistors, resistors and capacitors, a piezoelectric oscillating unit, a capacitance-variable diode, and a control voltage section formed by the resistors and the capacitors. In the voltage-controlled piezoelectric oscillator, the frequency starting characteristic of the piezoelectric oscillating circuit is shortened by varying a voltage of the anode end of the capacitance-variable diode with time. Patent Reference 2 discloses a voltage controlled oscillator, wherein a voltage-controlled oscillating circuit and a buffer circuit are serially connected, viewing from a power source. In the voltage controlled oscillator, constant voltage diodes are connected in parallel to the power source system of the oscillating circuit, and the oscillating frequency is stabilized regardless of the variation of the power source voltage.
Patent Reference 3 discloses a stabilized oscillation circuit, having frequency fluctuation suppression circuit 13a. In the frequency fluctuation suppression circuit 13a, by controlling a current of a differential amplifier type buffer output circuit 12 according to a power source voltage of a differential amplifier type oscillation circuit 11, a capacitance between the ground and the collector of the transistor of a differential amplifier pair in the differential amplifier type buffer output circuit is controlled, and the fluctuation of an oscillation frequency depending on the fluctuation of the power source voltage is suppressed.                [Patent Reference 1] JP 10-224148        [Patent Reference 2] JP 02-100502        [Patent Reference 3] JP 11-186843        
However, the conventional methods for compensating the frequency drift characteristic of the crystal oscillator have the problem that the frequency drift characteristic, which varies due to heat, cannot be properly compensated because the compensation amount is fixed.
In addition, FIG. 6, which will be further described hereinafter, is a diagram illustrating the frequency drift characteristic of a conventional temperature compensated crystal oscillator and shows that the variation of the frequency drift characteristic becomes larger as the power source voltage increases.
That is, as shown in FIG. 6 and FIG. 10, the frequency drift characteristic relates to the power consumption of the oscillator and also varies due to the power consumption. Accordingly, the frequency drift characteristic deteriorates as heat increases due to the variation of the power source voltage. In comparison with setting the power source voltage low, the frequency drift characteristic becomes worse when the power source voltage is set high. The conventional crystal oscillator cannot suppress such a problem.
In addition, considering the issue that power consumption changes due to the variation of the power source voltage and the frequency drift characteristic varies due to heat, the disclosures of Patent References 1 ˜3 do not compensate the frequency drift characteristic and stabilize the starting of the oscillator early.